No they are not the same.112.3 was the last stable version of Simutrans released some time last year. Since then many new features were added such as double height support and better climate model which were exposed in RC120. You will need RC120 for most new versions of pak sets such as the new pak128 with half heights.

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If not, is it stable?

It was meant to be stable but after extensive testing over the summer a number of critical faults were discovered. As such I have to strongly discourage the use of RC120 at the moment. That said a new stable release is expected in the next week or so.

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where can I find an installer for 112.3?

I would skip it for now and instead try out one of the nightly builds if possible. Those are a lot more stable than RC120 at the moment and have a lot of new features not available in 112.3. Using the nightly builds you will be able to use the latest version of pak128 which features awesome half-heights for more realistic railroads. If you just want to use pak64 then you will also have access to double heights and many critical end-game balance fixes regarding maglev and high speed trains. Normally I would not recommend a nightly build as they are often very unstable however since another release is being approached and the current release is old/unstable I encourage it for now.